This invention relates to 2'-fluoro-4"-trans-cyclohexyl-terphenyl derivatives, and more particularly to novel liquid crystal compositions including 2'-fluoro-4"-trans-cyclohexyl-terphenyl suitable for use in electro-optical displays.
Liquid crystal display devices utilize electro-optical effects possessed by liquid crystals. The liquid crystals used in these devices include a nematic phase, a cholesteric phase and a smectic phase. The most widely used display mode uses liquid crystals in the nematic phase and include a twisted nematic type (TN type), a dynamic scattering type, a guest-host type, and the like.
Liquid crystal display devices have several advantages, including for example, their small size and ability to be made thin; the device can be driven at low voltage with low power consumption; the liquid crystal is a light receiving elements, when a liquid crystal display is viewed over a long time period, eye strain does not occur.
Thus, liquid crystal display technology has been applied to watches, electronic counters, audio equipment and automobile dash board indicators, and the like. More particularly, liquid crystal devices have also been applied recently to personal computers and word processor displays and to other displays requiring high resolution and many pixels, including black and white and color pocket televisions, and the like. Thus, liquid crystal display devices have attracted attention as potentially replacing cathode ray tubes. As a result, liquid crystal display devices have been applied in various areas and it is likely that the use will be broadened further.
For practical use, liquid crystal compositions must possess the following characteristics:
1. Liquid crystal must be colorless and thermally, optically, electrically and chemically stable;
2. Have a wide temperature range and a wide viewing angle;
3. A rapid electro-optical response speed;
4. Require a low driving voltage;
5. A steep rise in voltage-light transmittance; and
6. The temperature dependency of threshold voltage be small; and,
7. A wide visual angle.
Many liquid crystals have one of the above desired properties, however, no compound satisfying more than one property is known. Thus, liquid crystal compositions are formed of several different nematic liquid crystal compounds or liquid crystal compositions are obtained by mixing liquid crystal compounds with non-liquid crystal compounds to obtain the desirable properties.
In general, a composition including a liquid crystal compound having a relatively low molecular weight and having C-N point (or melting point) at about room temperature and a liquid crystal compound having a high molecular weight and having C-N point higher than 200.C is used. In order to satisfy property 2, a liquid crystal compound having an N-I point as high as possible and a N-I point as low as possible is required. However, conventional liquid crystal compounds having an N-I point higher than 200.degree. C. do not have an N-I point higher than 250.degree. C., while their C-N point is relatively low, as shown in Table 1.
Conventional liquid crystal compounds having a high N-I point also have high viscosity. Thus, these liquid crystal compounds tend to increase the viscosity of the liquid crystal composition obtained by mixing them, resulting in a delay in the response speed of the liquid crystal display devices. In order to decrease the driving voltage of a liquid crystal display device, it is necessary to reduce the threshold voltage. However, the following relationship exists between threshold voltage (Vth), elasticity constant (K) and dielectric constant anisotropy (.DELTA..epsilon.): ##EQU1## Thus, in order to decrease V.sub.th, a liquid crystal compound having large .DELTA..epsilon. and small K is required. However, conventional compounds having a high N-I point and large .DELTA..epsilon. have a very large elasticity constant which results in an adverse increase in V.sub.th.
__________________________________________________________________________ Liquid Crystal CN Point NI Point Compound (.degree.C.) (.degree.C.) __________________________________________________________________________ ##STR2## 130 239 ##STR3## 94 219 ##STR4## 94.7 246.7 ##STR5## 100.5 231 __________________________________________________________________________
Accordingly, it is desirable to provide an improved liquid crystal material having a low C-N point, a high N-I point and a low viscosity.